Our goal is to understand the mechanism of interference mediated by defective (DI) influenza viruses. Recently we have seen RNA segments absent in cloned infectious viruses are present in DI viruses and appear a characteristic of DI viruses. Preliminary data suggest that these small RNA molecules (DI RNA) arise from the deletion of viral RNA segments and are the principal interfering molecule(s). We propose to develop a direct assay for quantitating DI viruses. Furthermore, we propose to characterize these DI viruses, determine their role in interference and elucidate processes involved in replication of these DI viruses. We would also like to determine the etiology of persistent infections and the role of DI influenza virus in viral persistence.